1. Field of the Invention
The invention relates mainly to a liquid crystal composition suitable for use in an active matrix (AM) device, and an AM device including the composition. The invention relates especially to a liquid crystal composition having a positive dielectric anisotropy, and to a device having a twisted nematic (TN) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode or a polymer sustained alignment (PSA) mode including the composition.
2. Related Art
In a liquid crystal display device, a classification based on an operating mode for liquid crystals includes phase change (PS), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA), and polymer sustained alignment (PSA). A classification based on a driving mode in the device includes a passive matrix (PM) and an active matrix (AM). The PM is further classified into static, multiplex and so forth and the AM is further classified into a thin film transistor (TFT), a metal-insulator-metal (MIM) and so forth. The TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high-temperature type and a low-temperature type according to the production process. A classification based on a light source includes a reflection type utilizing natural light, a transmission type utilizing a backlight, and a semi-transmission type utilizing both natural light and a backlight.
These devices include a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to give an AM devices having good general characteristics. Table 1 below summarizes the relationship between the general characteristics of the two. The general characteristics of the composition will be further explained based on a commercially available AM device. The temperature range of a nematic phase relates to the temperature range in which the device can be used. A desirable maximum temperature of the nematic phase is 70° C. or more and a desirable minimum temperature is −10° C. or less. The viscosity of the composition relates to the response time of the device. Rotational viscosity also relates to the response time. A short response time is desirable to display moving images on the device. Accordingly, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1General Characteristics of a LiquidCrystal Composition and an AM DeviceGeneral CharacteristicsGeneral Characteristics of anNoof a CompositionAM Device1Temperature range of aUsable temperature range isnematic phase is widewide2Viscosity is small1)Response time is short3Optical anisotropy isContrast ratio is largesuitable4Dielectric anisotropy isDriving voltage is low andpositively or negativelyelectric power consumption islargesmallContrast ratio is large5Specific resistance isVoltage holding ratio is largelargeand a contrast ratio is large6It is stable toService life is longultraviolet light and heat1)A liquid crystal composition can be injected into a liquid crystal cell in a shorter period of time.
The optical anisotropy the composition relates to the contrast ratio of the device. The product (Δn·d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed in order to maximize the contrast ratio. A suitable value of the product depends on the kind of operating modes. In a device having a TN mode or the like, a suitable value is about 0.45 μm. In this case, a composition having a large optical anisotropy is desirable for a device having a small cell gap. A large dielectric anisotropy of the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio. Accordingly, a large dielectric anisotropy is desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. Accordingly, a composition having a large specific resistance is desirable at a room temperature and also at a high temperature in the initial stage. A composition having a large specific resistance is desirable at room temperature and also at a high temperature after it has been used for a long time. The stability of the composition to ultraviolet light and heat relates to the service life of the liquid crystal display device. In the case where the stability is high, the device has a long service life. These characteristics are desirable for an AM device used in a liquid crystal projector, a liquid crystal television, and so forth.
Conventional compositions are disclosed in the following patent documents No. 1 to No. 6. No. 1: DE 4,006,921 A; No. 2: WO 1996-11897 A; No. 3: JP 2003-176251 A, and No. 4; WO 2005/019,377 A.
A desirable AM device has characteristics such as a wide temperature range usable, a short response time, a large contrast ratio, a low threshold voltage, a large voltage holding ratio and a long service life. Even one millisecond shorter response time is desirable. Thus, the composition having characteristics such as a high maximum temperature of a nematic phase, a low minimum temperature of a nematic phase, a small viscosity, a large optical anisotropy, a large dielectric anisotropy, a large specific resistance, a high stability to ultraviolet light and a high stability to heat is especially desirable.